1. Field of the Invention
This invention relates to a surface light source device comprising an aggregate of point light sources comprising light emitting elements such as an LED mainly used for a back light of a liquid crystal display panel, and an adjusting method of chromaticity thereof.
2. Description of Prior Art
An edge light type surface light source using a translucent flat plate as a light guide plate is well known as a surface light source for a backlight of a liquid crystal display panel. In such the surface light source, light is incident from one side end surface (an edge part) of a light guide plate of a translucent parallel flat plate or a wedge-shaped flat plate, and the incident light is diffused uniformly to an entire area of the light guide plate. A part of the diffused light is turned into diffused reflected light by a light reflecting member on a rear surface of the light guide plate, and the diffused light is emitted from a front surface of the light guide plate.
The light guide plate of the edge light type surface light source used as a light source of a liquid crystal display panel is widely used because it enables to reduce a thickness of the light guide plate and to simplify a structure of the light source.
The edge light type surface light source in general is provided with a cold cathode ray tube lamp on the edge part of the light guide plate as a light source.
Meantime, portable type equipment has become popular, and equipment consuming less power comes to be required. In conjunction with this, a light source using an LED element consuming less power as compared with a conventional cold cathode ray tube lamp has been paid attention and has been commercialized as a light source for a backlight of the portable type equipment.
FIG. 1 is a schematic cross sectional view illustrating a surface light source using an LED element. As shown in the figure, the surface light source comprises a light guide plate 1, a point light source 2 formed by an LED element.
A light source part 20 comprising a plurality of the point light sources 2 formed by white LED elements is arranged on an edge part of the light guide plate 1.
A light reflecting surface 11 is arranged on a side vertical to the point light source 2 of the light guide plate 1, and a light emitting surface 12 is arranged on an opposite side to the light reflecting surface 11. A white reflecting sheet 5 is arranged so as to face to the light reflecting surface 11, and light transmitting through the light reflecting surface 11 is reflected on the white reflecting sheet 5 and returned to the light guide plate 1.
The light emitting surface 12 is arranged on a side opposing to the light reflecting surface 11 (a front surface side). Light from the point light sources 2 is reflected on the light reflecting surface 11, three side surfaces 13 of the light guide plate 1, and the reflecting sheet 5, and most of the incident light is emitted from the light emitting surface 12.
A diffusing sheet 3 and a lens sheet 4 are arranged on an upper surface of the light emitting surface of the light guide plate 1.
A reflector 22 surrounds an LED substrate 21 mounting the LEDs as the plurality of point light sources 2 in the light source part 20, and light from the point light sources 2 is guided to the light guide plate 1.
Color tone is one of performance indexes of a surface light source device. Generally it is evaluated by a xy value in a xy chromaticity diagram. Color tone is evaluated by color tone at a center part of the light emitting surface of the surface light source device, and color tone unevenness in comparing those of the center part and a peripheral part.
One unit of a surface light source device using a cold cathode ray tube lamp includes one unit of the cold cathode ray tube lamp. Therefore, color tone unevenness on a light emitting surface is reduced and color tone difference between individual lamps is relatively small.
On the other hand, a surface light source device using an LED element as a light source uses a plurality of the LED elements. Furthermore, color tone between the individual LED elements is different. Therefore, it has disadvantage in color tone and color tone unevenness as compared with the surface light source device using a cold cathode ray tube lamp.
In general, a white LED element is categorized into a plurality of grades by luminous intensity and color tone and is shipped with information of what grade of luminous intensity and color tone the LED element was given. However, it is difficult to specify and purchase LED elements of the desired grade. The purchaser can only confirm the properties of the purchased LEDs. In some case, although it is possible to specify the grade of LEDs to be purchased, it may have disadvantage in cost and delivery.
FIG. 3 is one example of color tone grades of a white LED element. In this example, LED elements are categorized into three grades xe2x80x9ca, b, cxe2x80x9d in the xy chromaticity diagram. As compared with an LED in the xe2x80x9cbxe2x80x9d grade in center, the LED in the xe2x80x9caxe2x80x9d grade is bluey white, the LED in the xe2x80x9ccxe2x80x9d grade is yellowish white. A manufacturer gives this color tone grade with regard to distributions of white LED elements on a xy chromaticity diagram. More specific classification other than the three-group classification or other gradation are made in other cases.
When a surface light source device within a range of color tone indicated in a back light specification surrounded by a broken line in the figure, LED elements in the xe2x80x9cbxe2x80x9d grade are preferred in viewpoint of center color tone and evenness of color tone. However, when the device is only formed of only the xe2x80x9cbxe2x80x9d graded elements and the xe2x80x9caxe2x80x9d and xe2x80x9ccxe2x80x9d graded elements are disused. Manufacture cost increases as a whole.
This invention was made to solve these problems and provides a surface light source device capable of suppressing chromaticity difference even in using a point light source. For example, a surface light source device equivalent to the one comprising only the white LED elements of the xe2x80x9cbxe2x80x9d grade is provided even when the white LED elements of the xe2x80x9caxe2x80x9d and xe2x80x9ccxe2x80x9d grades are added to the xe2x80x9cbxe2x80x9d grade elements.
A surface light source device according to this invention comprises a light guide plate including a light incident surface and a light emitting surface, a light source part including a plurality of point light sources arranged on a light incident surface side of the light guide plate. Two of the point light sources of the light source part having chromaticity out of a center of an aimed chromaticity range in opposite hue directions are arranged adjacently.
The light incident surface is provided on one side end surface of the light guide plate, and the light emitting surface is provided on a front surface positioned in vertical direction to the one side end surface of the light guide plate.
A light reflecting member is provided on an opposite side of the light incident surface.
The light reflecting member is provided with chromaticity correcting means corresponding to emitted light.
The light source part is provided with not less than four point light sources, and two of the point light sources having chromaticity out of the center of the aimed chromaticity range in the opposite hue directions are arranged in an approximately center of a group of the plurality of the point light sources.
The two of the point light sources have the chromaticity outermost from the center of the aimed chromaticity range in the opposite hue directions.
The point light source comprises a white light emitting diode element.
The point light sources of different hues are arranged adjacently in the surface light source device of this invention. Therefore, colors of the point light sources are mixed and color tone turns into a medium one. Thus, the surface light source device capable of suppressing color tone difference as a whole can be provided.
An adjusting method of a surface light source device which light is incident from a plurality of point light sources arranged on a side of a light incident surface provided on a one side end surface of a light guide plate and is emitted from a light emitting surface on a front surface of the light guide plate comprises a process for arranging the plurality of the point light sources having chromaticity out of a center of an aimed chromaticity range in opposite hue directions adjacently, a process for providing measurement of light which is emitted from the point light source and further is emitted from the light guide plate and judging whether the light is in the aimed chromaticity range, and a process for arranging chromaticity correcting means corresponding to the emitted light in connection with an optical propagating path of the light guide plate when it is found from the measurement that the emitted light is not in the aimed chromaticity range.
With this method, the point light sources of different hues are arranged adjacently in the surface light source device of this invention, and colors of the point light sources are mixed and color tone turns into a medium one. Thus, the surface light source device capable of suppressing color tone difference as a whole can be provided, and the surface light source can satisfy an aimed chromaticity range. When the surface light source device out of the aimed chromaticity range can be modified in one having the aimed chromaticity range by arranging chromaticity correcting means in connection with an optical propagating path of the light guide plate.
An adjusting method of a surface light source device which light is incident from a light source arranged on a side of a light incident surface provided on a one side end surface of a light guide plate, is reflected on a reflecting member arranged in proximity to a rear surface opposite to a front surface of the light guide plate, and is emitted as a plane-shaped light from the front surface of the light guide plate comprises a process for providing measurement of chromaticity difference from an aimed chromaticity range of light which is emitted from the light source and further emitted from the light guide plate, and a process for selecting a colored reflecting member for correcting the emitted light to be in the aimed chromaticity range as a reflecting member arranged in proximity of the rear surface of the light guide plate.
With this method, a chromaticity difference from the aimed chromaticity range is measured and a colored reflecting member for correcting emitted light to be in the aimed chromaticity range is arranged in proximity of the rear surface of the light guide plate on the basis of the measured chromaticity difference.
The point light sources and the light source comprises a group of the point light sources including not less than four white light emitting diode elements, and two of the white light emitting diode elements having chromaticity outermost from a center of the aimed chromaticity range in opposite hue directions are arranged in an approximately center of the group of the point light sources.
The chromaticity correcting means is a colored reflecting member provided on a rear surface opposite to the front surface of the light guide plate for emitting light.